The dorsal cochlear nucelus (DCN) is an early auditory processing center that integrates auditory and nonauditory information. The role of nonauditory input to the DCN is not understood, but it may play a role in sound localization by the DCN and in certain forms of the auditory disorder tinnitus. Nonauditory input to the DCN is relayed to the principal cells of the DCN by granule cells. Granule cells are subject to prominent inhibition from Golgi cells, making Golgi cells well situated to control the flow of nonauditory information between granule cells and principal cells. I have identified two novel types of neuromodulators, cholingeric and type 2 metabotropic glutamate (mGluR2) receptor agonists, that decrease the size of IPSCs at the Golgi-to- granule cell synapse. Anatomical studies suggest that glutamatergic inputs likely to activate Golgi cell mGluR2 receptors carry non-auditory information, whereas cholinergic fibers may carry indirect auditory input. Thus, the Golgi-to-granule cell synapse may be dynamically regulated in response to nonauditory and auditory input. The goal of this proposal is to determine the key functional properties of the Golgi-to-granule cell synapse and how these properties are regulated by release of endogenous neuromodulators. The experiments in Aim 1 will test the hypothesis that the Golgi-to-granule cell synapse is a low release probability (Pr) synapse and that neuromodulators inhibit this synapse through reducing Pr and the number of release sites. Multiple probability fluctuation analysis will be used to determine the synaptic parameters of the Golgi-to-granule synapse and how these parameters are changed by cholinergic and mGluR2 agonists. The effect of cholinergic and mGluR2 agonists on Ca2+ influx in Golgi cell synaptic boutons will be examined using 2-photon microscopy. Data obtained during trains of IPSCs at the Golgi-to-granule cell synapse will be used to develop a model for short- term synaptic plasticity at this synapse. The experiments in Aim 2 will examine whether release of endogenous neuromodulators can regulate the Golgi-to-granule cell synapse and will test the hypothesis that concurrent release of acetylcholine and glutamate has a supralinear effect on inhibition of IPSCs at this synapse. Using optogenetics to stimulate cholinergic fibers and extracellular stimulation to activate mossy fibers, the effect of release of endogenous acetylcholine and glutamate on IPSCs at the Golgi-to-granule cell synapse will be examined. The number of Ca2+ channels required to open to trigger release of a vesicle at the Golgi-to-granule cell synapse is a major determinant of whether concurrent release of endogenous acetylcholine and glutamate, as likely occurs in vivo, has a linear or supralinear effect on inhibition of IPSCs at this synapse. Membrane- permeant Ca2+ chelators will be used to provide a qualitative estimate of the number of Ca2+ channels required to open to trigger release at the Golgi-to-granule cell synapse. Lastly, the effect of concurrent release o endogenous acetylcholine and glutamate on the Golgi-to-granule cell synapse will be determined.